1. Field of the Invention
The invention relates to a method for in-line measuring the active KOH concentration in a KOH etching process in which process silicon hydroxide is produced by a reduction reaction according to the formula:2K+ (aq.)+20H− (aq.)+2H20+Si→2K+ (aq.)+H2SiO42− (aq.)+2H2 (g).
2. Related Art
The actual etching reactions in KOH etching are non-trivial complicated. The reaction produces silicon hydroxide in the etching process by a reduction reaction. Silicon hydroxide, however, is not a stable compound, and it is transformed into a silicon complex. One molecule of hydrogen per one atom of silicon etched is released in the process.
The hydrogen silicate ions H2SiO42− in the solution tend to polymerize, which sometimes leads to a ‘slimy’ appearance of the reaction products, and makes determining the actual reactions even more difficult.
For the sake of simplicity the reaction shown above is used here to describe the .process and the weight/weight concentration values shown later in the text paper are calculated by using the molar masses of potassium hydroxide (KOH, 56.11 g) and dipotassium hydrogen silicate (K2H2SiO4, 172.30 g).
The etch rate of silicon in a KOH bath depends on the bath temperature and the KOH concentration. Both parameters have to be measured in order to obtain a sufficiently accurate estimate of the etching time.